$ \left(\dfrac{7}{9}\right)^{-3}$
Answer: $= \left(\dfrac{9}{7}\right)^{3}$ $= \left(\dfrac{9}{7}\right)\cdot\left(\dfrac{9}{7}\right)\cdot \left(\dfrac{9}{7}\right)$ $= \dfrac{81}{49}\cdot\left(\dfrac{9}{7}\right)$ $= \dfrac{729}{343}$